1. Introduction to Python
Lecture 5
Kasyanov Anton
IASA 2011

2. Plan
 Classes
 Tips

3. What are classes?
 Python allows us to build our own types.
 These are called classes.
 When we have a class, we can create
instances/objects of that class.
 Recall the difference between a type (str, int)
and a value ('this is a string', 10).
 This is analogous to the difference between a
class and an object.

4. Classes
 To make a class we just do:
class Class_name(object):
block
 Class is a keyword and object is a type.
 Class names start with Capital letters by
convention.
 To create objects or instances of Class_name
we use:
x = Class_name()

5. Methods
 To create a method we use the keyword self
 Say we have a class Patient that is meant to
store information about patients in a hospital.
 We may want to update the patient's age.
 To do this we need a method set_age.
class Patient(object):
def set_age(self, age):
self.age = age

6. Methods
 Note that these are methods, so that if we have
a patient p1 to set his age we use
p1.set_age(10) and not
p1.set_age(p1,10)
 Self should always be the first parameter in a
method but it is passed automatically when we
call the method.

7. Constructors
 There is also a way to set class variables when
you construct class instances.
 The special method __init__ is called whenever
you try and construct an instance of an object.
class Patient(object):
def __init__(self, name, age):
self.name = name
self.age = age
 Called by x = Patient(“Joey”, 10)

8. Constructors
 Using constructors we can also init variables:
 def __init__(self):
 self.data = []

9. Builtin methods
 __ indicates that the method is a special
method.
 These are used to make our classes work more
like Python's built-in types.
 For example:
 __str__ is used when printing
 __cmp__ is used to allow boolean operations.
 __add__ is used to allow the + operator.
 __iter__ is used to allow your type to be used in
for loops.


10. Structure
 If we need just a simple structure, we can use
this:
 class Employee:
 pass
john = Employee() # Create an empty
employee record
 # Fill the fields of the record
 john.name = 'John Doe'
 john.dept = 'computer lab'
 john.salary = 1000

11. Naming conventions
 Class names start with upper case letters.
 Class methods and instances start with lower
case letters.
 Method definitions should have docstrings just
like function definitions.
 Classes should have docstrings just like
modules have docstrings that describe what the
class does.

12. Inheritance
 The syntax for creating subclasses is:
 class Class_name(Superclass_name):
block
 Note that this means our previous class is a
subclass of the class object.
 If you define a method with the same name as
one in the superclass, you overwrite it.

13. Lambda functions
 Lambda functions are very simple one-line
functions that can be treated as an expression.
 Syntax: lambda arg1,arg2,... : expr

 def add(a,b): return a+b
 add2 = lambda a,b: a+b

14. List comprehensions
 One line way to create a list
 E.g.: [a*2 for a in [1,2,3]]
 Returns:
 [2,4,6]

15. Summary
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